Electric induction furnace



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ELECTRIC INDUCT ION FURNACE Filed Jan. 5, 1931 2 Sheets-Sheet 1 I BMW ib-m if a Jim/law vmvawwww Nov. 20, 1934. E N RTH RUP 1,981,631

ELECTRIC INDUCTI ON FURNACE Filed Jan. 5, 1931 2 Sheets-Sheet 2 Patented Nov. 20, 1934 PATENT OFFICE UNITED STATES,

, 1,901,031 v ELECTRIC INDUCTION FURNACE Edwin Fitch Northrup, Prlnceton,'-N. J., aasignor Ajax Electrothermic Corporation, Park, Ewing Township,

New Jersey Ajax N. J., a. corporation of Application January 5,1931, Serial No. 506,5'l1

Claims. (Cl. 219-13) My invention relates to electric induction furnaces and'to methods of controlling their operation.

A purpose of my invention is to predetermine 5 the rate of inductive heating of charges in ac-.

temperatures more rapidly than those coming at A further purpose is automatically .to alter the number of inductor coil turns connected to the source with change in the temperature differential between the initial and final charge tem-v peratures.

Further purposes appear in the specification and in the claims. I

My invention relates chiefly to the methods involved, but it also includes apparatus suitable ,to carry out the methods.

In the drawings I illustrate one general type of apparatus, of the many which might be employed, as well as one specific instance of the general type. I have chosen the apparatus largely from the standpoint of convenient illustration of the principles involved, and with the knowledge that many others might be devised. Figure 1 is a chart indicating steps in my method.

Figure 2 is a diagrammatic section of a general form of apparatus which I may employ.

Figure 3 is a diagrammatic perspective view of a specific circuit which I may conveniently use.

In the drawings like numerals refer to like parts.

In metallurgy it is very commonto heat metallic charges to relatively high temperatures, to subject them to operations during which they cool to an uncertain extent, and finally to reheat them to a desirably definite temperature, usually determined by one of the critical points of the particular metal.

The making of iron and steel is replete with cases in which the metal, at an uncertain relatively high temperature, must be heated to a definite temperature. I will refer briefly to a few instances.

Iron from the blast furnace is commonly tapped into ladies and carried directly to the steel-making furnace without intermediate solidification. Various charges enter the steel-making furnace, as for example an open hearth, at variant temperatures, depending among other things, upon the temperature at which the blast furnaceis tapped, the size of the ladle and the length of time in which the steel is held in the ladle.

However, it is important to tap the steelmaking furnace at a definite temperature without regard to the charging temperature of the metal. As a consequence, the cycle of operation of the steel-making furnace varies with the individual, charge, in the effort of the operator to obtain a uniform finishing temperature.

Again, the product of the open hearth is frequently charged into an electric induction furnace and there further refined or mixed with alloying ingredients. The initial temperature of the induction furnace charge is again dependent upon a number of factors, such as the temperature of tapping of the open hearth, the size of the ladle, the time during which the steel is held in the ladle and the time during which the induction furnace has been idle prior to charging.

In order to obtain a uniform pouring temperature for the induction furnace, the time of the heating operation is varied to suit the needs of the particular charge. Y

After the ingots have been poured, have solidifled and have been removed from the mold by stripping, they are ordinarily placed in a soaking pit, while they still retain some sensible heat from the steel-making furnace. The temperatures of the ingots entering the soaking pit vary considerably, and therefore ordinarily the ingots are allowed to remain in the soaking pit for such a long time that it is certain that even those charged at the minimum temperature have reached the selected finishing temperature.

After the ingots have been removed from the soaking pit and have been cropped, they may be mechanically worked, as for example by rolling, and, after several passes through the rolls, placed. in a reheating furnace. Here also, the temperatures of the charges as they enter the reheating furnace vary widely, and the reheating operation must be lengthened considerably to make sure, that even those charges supplied at minimum temperatures have reached the proper finishing temperature for further mechanical working.

After mechanical working has been finished, the charges are commonly heated to a carefully regulated temperature and heat treatedyas by annealing or quenching. The temperature of charging into the'heat treating furnace varies, and the cycle of operation of the heat treating fumacevaries undesirably with individual charges.

Each of the instances cited, the open hearth, the induction furnace, the soaking pit, the reheating furnace and the heat treating furnace, presents the problem of regulating the operation to bring charges inserted at varying temperatures to a finishing temperature which is uniform from charge tocharge and from cycle to cycle.

With a rate of power input varying about in proportion to the required increase in tempera ture it is possible to make the time of heating so nearly uniform for all charges (independently of the initial temperature)v that workmen can proceed confidently upon a time basis or schedule without having to verify temperatures.

In as much as each of the types of furnaces named, andinfactall furnaces operating upon metallic charges and many .handling nonmetal- .lic charges, may now beor may be in the future an induction furnace, my invention is .broadly' applicable to all of them.

'Inherently my invention is not limited to use upon molten or upon solid charges, although I anticipate that its widest application will be in heating solid char es for the purpose of mechan-' ical working or hea treatin'g'.

I have discovered that the operation of aninduction electric-furnace may be very desirably improved by employing a cycle of constant length and automatically varying the power input of the furnace inversely with respect to the initial temperature of the individual. charge. If an individual charge be at a relatively low initial temperature, I will apply a higher power input, so that it will reach the required temperature by the end of the predetermined time, while if it be. at a relatively high initial temperature, I will use a comparatively low power input, and still finish heating in the same time. '1' Thus with a charge of low initial temperature the-power input is relatively high, whilewith one of high initial temperature the power input is'relatively low. Although no proportionality exists between the initial temperature. of the charge and the power input,.the power input varies inversely with respect to the initial temperature'. i

Essentially my invention comprises an inductor coil connected to a power source, suitable power switches and connections for changing the pow er input to the inductor coil, a temperature measi urer for determining the initial charge tempera-' ture and a powerswitch control responsive to the temperature measurement for-varying the power input iBYet ely with respect to the a temperature measurement. The location of the *tem-- e perature measurer, whether in the induction rm,

nace or in a separate place, is relatively immaterial to my invention. r

v In Figure 1 I illustrate three steps proceeding in time sequence from left to right. The charge is first'heated in a preliminary furnace 20, is subsequently exposed to the-predeterminer 21, and finally passes to the induction furnace 22.

The preliminary furnace may be any furnace whatever in'which heat is added to the charge, as for example a blastfurnace open hearth, induction furnace, soaking pit or reheating furnace. The function of the preliminary furnace from the standpoint of my invention is solely to add heat to the charge, without regard to whether the charge be melted or subjected-to any other operation. Ordinarily heating in the preliminary furnace is incidental to some other purpose there being served.

Between the preliminary furnace and the predeterminer there may be any number of steps, as

.indicated by the dotted fiow line 23, during which I then place the charge in the redeterminer,

where its temperature is, gaged a d the power input of the induction furnace is regulated according to the gaged temperature. Between the predeterminer, and the induction furnace there is no temperature change or a definite temperature change of the charge, as indicated by the solid flow line 24.

I refer to the operation ias gaging ratherthan measuring", notwithsilzanding that all of the physical steps of measu ing are performed, because the knowledge of the temperature of the charge need not be brought home to the operator, since the variation in the power input is automatic.

The predeterminer may be in the induction furnace itself. in which case. the temperature which it gages is the temperature at which the inductive heating starts; In this event the pre determiner must be disconnected after thefurnace starts to heat the charge. On the other hand, the predeterminer may be located at some distance from the induction furnace, and all charges passing from the predeterminer to the induction furnace will in that case undergo the same conditions so that the temperature gaged in the prede terminer will have a definite relation to the initial temperature of. the charges in the induction furnace.

The particular mechanism for carrying out my invention is of relatively secondary importance,

as many diifer ent forms of apparatus could be devised, I

In Figure 21 show operation upon metallic pipe. The predeterminer 21' having-a wall 25 is located at a point earlier in the direction of charge travel than the induction furnace 22, and is connected to the power variation switches of, the induction fumace so that the power input is 'regu-- lated inversely with respect to the temperature determination of the predeterminer.

A charge 26, is shown in the predeterminer rest- ,ing upon suitable supports, desirably consisting of rollers 27' and 28. A temperature gage 29 of any suitable character is exposed to the'heat of 5 2 v 1,981,681 the charge so that it gives a relative determinabe gaged and the conditions in the predeterminer.

For example, the temperature gage 29 may operate by variation of electrical resistance, by change in thermal'electromotive force, by alteration in photoelectric current, by thermal expan-' sion, or in any other manner. Of course, with each ofthese forms the usual circuit will be used; with suitable resistances, reactances, power sources, relays or meters as required. I do not refer to the gage as a "pyrometer because I do not wish to limit myself to high temperatures within the pyrometer range, although the gage will ordinarily be required to operate on high temperatures.

In Figure 2 I show conventional connections 30 and 31 through contacts 32 and 33 to a relay switch 34 responsive to variation in the electrical characteristics of the circuit from the temperature gage 29. The contacts 32 and 33 are closed when v the charge 26 is in the predeterminer and open,

as for example by upward spring of the contact .37, 38,39, 40, 41, 42, 43, 44 and 45, which are connected by leads 46, 47, 48, 49, 50, 51, 52, 53, 54 and 55 to remote control coils 56, 5'7, 58, 59, 60, 61, 62, 63, 64 and-65. The circuit through the remote control coils from a source 66 is completed by a connection 67 which is common to each of the remote control coils.

Each of the individual remote control coils closes one of the switches 68, 69, '70, 71, 72, 73, 74, 75, 76 and 77, which impress the voltageof an alternating power source 78 across varying numbers of turns of the inductor coil '79 through suitable taps. The power factor of the inductor coil is corrected by capacity shunted across the entire coil. The inductor coil is indicated as hollow flattened edgewound water cooled tubing, but of course it may be of any suitable type.

In operation, when the charge 26 is inserted in the predeterminer 21, the switch contacts 32 and 33 are closed so that the temperature gage 29.is operative to control the relay switch 34. If the individual charge 26 inthe predeterminer be at an abnormally low temperature, a relatively small current flows through the connections 30 and 31 or a relatively low voltage is impressed across them and the relay switch arm 35 finds a position near the end 45 of the switch arm movement.

Assume that contact 'is-made at 45. Then the remote control coil 56 is excited, closing the switch 68, and impressing the voltage of the power source across only a small part of the inductor' coil.

source are the primary and the entire coil, connected across the capacity 80, is the secondary. With the power source '78 connected across only a small part of the inductor, the ratio of transformation is high and the voltage impressed,

across the capacity is correspondingly high, so that the oscillation circuit carries a large current. When the charge 26 moves to the position 26' indicated in dot and dash, it is subjected to a high power input which heats it relatively rapid-- ly. Since, however, its initial temperature was low, it requires a relatively large amount of heat, and is brought to finishing temperature in exactly the time chosen for completion of the cycle.-

If, on the other hand, the charge 26 be at a relatively high temperature when placed in the predeterminer, a high current flows in the connections 30 and 31'0r a large voltage is impressed across them, moving the relayswitch arm 35 to a position near the end 36 of the contacts. Assume that contact is made at 36. Then the remote control coil 65 is excited, closing the switch 77 and connecting the power source 78 across the entire inductor coil. The ratio of transformation of the inductor coil is low, producing a correspondingly low oscillation circuit current and low power input. This is desirable because the charge came to the induction furnace at an abnormally high temperature. The charge is brought to the uniform finishing temperature in the same time as was required forthe charge of abnormally low initial temperature.

Charges entering the predeterminer at intermediate temperatures are correspondingly subjected to intermediate power inputs, so that all charges are finished in the same time.

In Figure 2 the charge '26 is evidently at a relatively high initial temperature, since the switch arm 35 at the point 37'has actuated the remote control coil 64 to close the switch 76, placing the inductor coil on relatively low power input.

The steps from'tap to tap need not represent equal temperature differences and the coils thrown in by reason of their successive engagement need not be equal in their additions to the heat input.

The number of taps from the inductor coil depends of course upon the number of turns in the inductorcoil and upon the closeness of regulation which is desired. For moderate size furnaces I find that 20 taps are sufficient, although any number could be used. Y

It will be evident that my invention could be carried out by placing the temperature gage in the induction furnace itself. However, I do not consider this as desirable as the formof Figure 2, because it would then be necessary to operate the power switches when the furnace ww under load, whereas now the switches are operated while the charge is-in the predeterminer and before the furnace comes under load. The oscillation circuit through the condenser 80 does not include the taps or switches 68 to 77, so that this equipment does not carry the heavy oscillation currents.

' The switch between the contacts 32 and 33 pre- 'Ihe circuit of Figure 2 is of'course entirely diagrammatic since the details of the temperature gage, the relay switch and the remote control switches are not shown. In Figure 3 I indicate in more detail one desirable form of my invention.

The charge 26 is shown in the predeterminer 21 which consists of a temperature gage 29' formed of, a continuous coil of flat wound nickel strip. To illustrate that the predeterminer will normally be of the approximate length of the charge, where the entire charge is to be heated,

or of the portion of the charge to be heated, where.-

only part of the charge is to be heated, the coil is conventionally broken in Figure 3.

The ends of the temperature gage 29' form one leg of an automatic self-balancing potentiometer system having fixed resistances 81 and 82 and. a

' resistance 83 varied by a'movable contact 84 on 7 condition is maintained automatically by means of a double deflecting galvanometer 96 connected to the point 94 by a lead 97 and to the point 95 by a lead 98. The galvanometer96 has a suitable electrically conducting pointer 99,. which;

' when the bridge is balanced, lies immediately below the block 100 of insulating material. The

pointer 99 is in reality a switch arm.

When the bridge is out of balance; current flows through the galvanometer 96,. and the pointer 99 swings either to the left or to the right, depending upon the direction of current fiow. If it swing to the left, it lies immediately'below the contact 101,

while if it swing to the right it lies immediately below the contact 102. The contact 101 is connected to a field winding 103 of the motor 87,

miner.

while the contact 102 is connected to an opposite field winding 104 of the same motor. The opposite ends of the field windings are joined to a lead 105 from a suitable power source 106.

Theopposite side of the power. source 106 is connected at 107 to one contact 33' of a switch closed by the charge when it is in. the predeter The other contact 32". of the switch is connected at 108 to an armature 109. The ar-' mature 109 constantly moves upand down under the influence of a solenoid 110 connected to-a power source 111. Whenthe armature is' in up'-' per position, the solenoid circuit is broken by separation of the contact 112 on the arma-i ture from the stationary contact 113, and the armature then drops until thesolenoid circuit is completed once more, when the armature receives a new impulse upward.

If the pointer 99 is below the block of insulation 100, the-motor field circuit is not completed by upward movement of the armature 109. If the pointer is below the contact 101. due to-unbalance of the bridge in one direction, the circuit through the field coil.103 is completed, and the motor is driven in the direction to cause the sliding con-- tact 84 to balance the. bridge; On the other hand, it the pointer is below the contact 102, due to imbalance of the bridge in the opposite direction,

the circuit through the-opposite field coil 104 is completed, and the motor turns in thereverse direction to balance the bridge.

- It, willfbe recognized of course that theautomatic self-balancing potentiometer has in practice many refinements not shown in my diagrammatic view. These are well known to electrical engineers, and may be seen on the Leeds and Northrup instrument of this type, or in a Leeds and Northrup catalogue forming part of the literature of this subject.

The nut 85 moves along the resistance 83 as the shaft 86 rotates, taking a direction depending upon the direction of rotation of the shaft. As it moves to balance the bridge, the. nut 85 also carries a power-switch arm 114, which travels over tap contacts 115, 116, 117, 118, 119, 120, 121, 122 and 123 of a secondary 124 of a transformer having a primary125 connected to an alternating current power source 126. One end of the secondary 124 is desirably connected at. 127 to one end of the inductor coil 79' which desirably consists of hollow flattened edgewound water cooled tubing; The inductorcoil is conventionally broken to indicate increased length to correspond roughly with the length'ot the charge 26', or of the portion of the charge to be heated, where less than the entire charge is to be operated upon. The other end of the inductor coil is connected at 128 to the power'switch arm 114. The power factor is desirably corrected by capacity 129 shunted across the inductor coil so that the oscillation circuit does not include the power switch arm 114 and the taps.

, At the end of the uniform heating cycle, the charge is withdrawn from'the inductor by any suitable means, such as the automatically operated tongs 130, which may be controlled by suitabletiming mechanism, or manually. 1

'When the predetermined winding29' is cool, it is of relatively low resistance, but, as soon as a charge is placed in the predeterminer, the winding becomes heated, and as it does so its resist- V .field of themotor 87 when the bridge is out of balance. Initially, since the resistance of the. temperature gage 29' is low at first, the bridge is balanced with the switch arm 84 near the point 94.

As the resistance of the predeterminer winding 29 increases with increase in temperature, the switch arm 84 moves'nearer to the point 90 in order to maintain the balance of the bridge. When the temperature gage 29' reaches an equilibrium temperature for the individual charge, the switch arm 84 will be close to or far from the point 90, depending upon whether the tem- 130 peratureof the individual charge be abnormally high 'or low. a

The-power'switch arm 114 will correspond inposition to the position of final balance of the switch arm 84. If the charge be at an abnormally high initial temperature, the temperature age 29' will have ahigh resistance, thebridge will be finallyv balanced with the switch arm 84 near the end 90, the powerswitch arm 114 will come to rest in contactwith one of the points 115, 116 or 117, a low voltage will be impressed across the inductor coil, and the furnace-will operate on low power, as it should because of the abnormally h h temperature of the charge.

Ii!v the charge be at a relatively low initial temperature, the temperature gage 29' will have a low resistance, the bridge wtll be balanced with the switch. arm 84 near the end 94, the power switch am 114 will then be in contact with one of the points .121, 122 or 123, a high voltage will be impressed upon the inductor coil and the furnace will be on high power. This is properbecause the initial temperature of the charge was low.

It will be evident of course that the number of turns included between each tap, the number of taps, and'the number of turns of the transformer secondary invariably left in the circuit because of location at the untapped end 131 of the transformer secondary are details of design merely.

The frequency which I will use depends of course upon the size of the furnace, the magnetic or nonmagnetic character of the charge, the

temperature of heating if the charge be normally- I believe that I am the first to heat charges of varying initial temperatures automatically to a uniform final temperature in an induction electric furnace. v

I further believe that Iam the first to inductively heat charges of uncertain initial temperatures to a uniform finishing temperature in a definite time.

I also believe that it is new to adjust the power in a high frequency furnace automatically.

In view of my invention and disclosure varivations and modifications to meet individual whim or particular need will doubtless become evident to others skilled in the art, to obtain part or all of the benefits of my invention without copying the structure shown, and I, therefore, claim all suchin so far as they fall within the reasonable spirit and scope of my invention.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent is 1. In the inductive heating of charges, means 'for determining the temperature range through ultimate temperature, an induction electric fur nace and means for regulating the induction furnace input automatically in response to variation in the difference between the compared temperatures.

3. In an electric induction furnace, an alternating current source, on inductor coiL'power switching connections between the source and the-inductor coil for varying power input to the inductor coil, a temperature gage for deter- =mining the relation of the temperature of an individual charge to an intended final ture and a control for the power switching connections responsive to change in the temperature relation.

.in the connections for varying the power input to the inductor coil, a temperature gage for determining the relation of the temperature of an individual charge to an intended final temperature and a control for the switches adapted to operate a switch decreasing the power input to the charge with decrease in the temperature difference.

5. In an electric induction furnace, an inductor coil, a source of alternating current, connections between the inductor coil' and the source, power switches in the connections for varying the power input to the inductor coil, a temperature responsive device for determining the relation of the charge temperature to an intended final temperature, a relay switch controlled by the temperature responsive device, a relay circuit having individual branches corresponding to contacts upon the relay switch, a source of current for the relay circuit and remote controls in the relay circuit, operating the respective power switches when energized by the relay circuit current.

6. In an induction electric furnace, an inductor coil, an alternating current source, connections between the inductor coil and the source, power switches in the connections adapted to open and close to vary the rate of inductiye heating of a charge, a temperature responsive device for determining the relation between the charge temperature and an intended final temperature, a control closing individual pow-er circuit switches in response to variation inthe temperature relation and a switch rendering the control inoperative to'vary the power input-to the charge when a charge is not in position for determination of.

- between the inductor coil and the source, power switches in the connections for varyingthe rate of inductive heating of a charge, a temperature responsive device for determining the relation between the charge temperature and an intended final temperature, a control operating individual power switches in response to the temperature relation and capacitative reactance across the entire inductor coil free from connection through the power switches.

8. In an electric induction furnace, aninductor coil, a. sou-roe of alternating current, connections between the source and the inductor coil, power switches in the connections for varying the power input to the inductor coil, 2. temperature responsive device for determining the relation between the charge temperature and an intended final temperature, an automatic self-balancingpotentiometer having a bridge, connections for the temperature responsive device in series with one resistance branch of the potentiometer bridge and a movable balancing arm for the bridge changing the power switch settings during its movement.

9. In an electric induction furnace, an inductor coil, a source of alternating current, a transformer comprising a primary and a secondary having taps, connections from the source to the primary, connections from the secondary to the inductor coil, a power switch in the connections, making contact in its various positions with individual taps, a temperature responsive device for determining the relation between the charge charge temperature and an intended final tempemimre, a control for changing the power input variation means in response to alteration in the temperature. relation and means for automatioazlly withdrawing the charge from the inductor coil at the end of an estgblished period of heat- EDWIN FITCH NORTHRUP. 

